This invention relates generally to main boiler superheater headers, and more particularly to preventing thermal shock to such headers by the initial introduction of steam to them.
In a typical main boiler design the steam from the main boiler is passed to a superheater which raises the steam temperature. The superheater itself is generally made up of an inlet header, superheater tubes located close to the main boiler flame, and an outlet header. Steam from the boiler flows into the inlet header until it reaches a division plate which prevents its further flow through the header and forces the steam into the tubes. When the steam reaches the outlet headers, a division plate there forces it back to the inlet header via other superheater tubes. By the use of the division plates the steam can be forced to pass back and forth for several passes in the superheater tubes in which the temperature of steam in the tubes is increased or "superheated" to a specific temperature at which the steam exit the superheater at the outlet header and proceeds to whatever apparatus will use the steam to accomplish work.
In the operation of a main boiler it generally requires some time after light-off of the boiler flame before sufficient steam is generated to flow through the superheater. In the meantime, the superheater tubes, which are located quite close to the flame, are subjected to extreme temperatures. To help prevent warpage of the tubes, it has been found useful to introduce "protection steam" from an auxillary source to run through the superheater until the main boiler generates enough of its own steam to establish an adequate flow through the superheater.
A problem which has been found to exist is a high degree of superheater header cracking, particularly in 600 p.s.i. and above main boilers. This problem is not confined to the particular boiler design nor to one boiler manufacturer. Many causes have been suggested for contributing to this problem, including: thermal shock, header design, high "locked-in" stresses, improper drainage, improper expansion, and improper protection steam piping. In addition, it is generally believed that the use of protection steam while protecting the superheater tubes actually increases the tendency toward header cracking.
The reduction of thermal shock caused by the initial introduction of steam into the header is a critical factor which must be controlled to alleviate header cracking. This is particularly true in the superheater inlet header, in the first pass region, where the highest initial transients are experienced during cold-iron light-off (i.e. when the boiler and header were initially at room temperature). It has been estimated that these initial transients may be as high as 400.degree. F within thirty seconds.
The prior art in the area of superheater control has been limited to stabilization of the temperature of the steam after its introduction into the superheater. Such temperature stability has been achieved by methods such as mixing the superheater steam with anxiliary steam, introducing temperature rasing material (e.g. magnesuim particles) into the superheater, and controlling the boiler air-fuel mixture. Although such temperature control is useful for several reasons, it fails to alleviate the thermal shock caused by the initial intorduction of steam into the superheater.